How to prevent legs from stretching when moving other parts of body?
There are some tips to fix your problem.


subdivide
weight paint
preserve volume


1. subdivide

For a "good" rigging, your model seems to be too low poly. I suggest you add some loop cuts (near the joint) or add a subdivision surface modifier above the armature modifier.
Make sure that your new vertecies have appropriate vertex weights. 

2. weight paint

By using weight painting, you can manipulate "the influence of the bone on a certain vertex". If some of your vertecies seem to move too much, decrease the vertex weight, mainly at the vertex paint mode.

3. preserve volume

At the Armature modifier panel, there is a check box labeled "preseve volume". Ticking it may improve your armature deform.